This invention relates to semiconductor lasers and, more particularly, to a Distributed-Bragg-Reflector semiconductor laser comprising, a nipinipi . . . gain-producing active section comprised of a plurality of gain segments disposed at periodic intervals with respect to the wavelength of an intended operating frequency of the laser; a pair of Distributed-Bragg-Reflector stacks disposed at the respective ends of the nipinipi . . . active section; and, electrode means operably connected to the nipinipi . . . active section in electrical contact therewith for applying a driving current to the nipinipi . . . active section.
Diode lasers are being used increasingly in such applications as communications, sensing, and so-called optical computing. A typical diode laser is shown in FIG. 1 where it is generally indicated as 10. The laser 10 has a mirror 12 at each end. A light beam 14 is created and amplified within the laser 10 by the pumping current source 16 operably connected thereto. The light beam 14 is created by a process termed "spontaneous emission" over a bandwidth around some center frequency. Some of this light is captured by the dielectric waveguide formed by the active medium. The guided light is reflected by mirrors 12 that create a Fabry-Perot resonant cavity. The modes of this resonant cavity are spaced nearly equally in frequency according to .DELTA.f=c/(2n.sub.g 1), where n.sub.g is the group index of the waveguide and 1 is the mirror spacing. Simply put, these mode frequencies are those for which 1 is an integer number of half wavelengths, or those at which the lightwave 14 adds constructively to itself after traversing the roundtrip of the cavity. Likewise, the mirrors 12 do not reflect 100% of the light striking them. As a consequence, there is some loss through the mirrors 12 at each reflection by the light beam 14. Also, there is propagation loss in the waveguide between the mirrors 12. As energy is added to the system by the current source 16, those frequency components of the light beam 14 which are in phase with the spacing of the mirrors 12 (i.e. Fabry-Perot modes) tend to be additive while those which are out of phase have components which tend to cancel out. The additive components continue to build in power until the laser "lases". That occurs at the current where the gain of the active medium equals the losses of the cavity waveguide and mirrors. Above this "threshold" current, the output laser light 18 from laser 10 increases rapidly in value.
As shown in FIG. 2, one can make a two part diode laser 20 by replacing one of the mirrors 12 with, for example, a grating 22. The grating 22 provides a multiple reflective surface at the one end such that there are multiple spacings between the single mirror 12 and the multiple reflective surfaces of the grating 22. Thus, there are fewer frequency bands in the emitted laser light 18.
In the growing field of optical communications and computing (and in similar applications), requirements of the total technique require the ability to form diode lasers as part of the overall computer structure and its interconnections on a single chip. As can be appreciated, such integrated optoelectronics must be highly accurate (to "aim" the laser beam produced from stage to stage on the circuit) and must have highly polished (i.e. "minimally damaged") surfaces associated therewith to minimize optical losses which could make the circuitry inoperative. Some of the first reactive etching efforts relative to optoelectronics were for the construction of laser facets and narrow etched grooves to create coupled-cavity laser structures similar to the surface-emitting structure depicted in FIG. 3. To get the power required from the laser 10, it is formed horizontally into the surface 24 of the semiconductor material 26 (i.e. inplane) in substantially the same manner as the other components of the circuit. The laser mirror facets are etched by critical etching techniques known in the art. To get the laser light 18 to emerge from the surface 24, a reflective facet 28 must be formed adjacent the point of emergence from the laser 20 so as to reflect the laser light 18 in the desired direction. As can be appreciated, high resolution pattern transfer and smooth vertical laser facets are required for efficient reflection of optical signals in optoelectronic integrated circuits (OEICs). Also, and more important, an in-plane laser such as depicted in FIG. 3 occupies a lot of "real estate"; that is, it occupies valuable space on the surface 24 of the semiconductor material 26 that could be used for a multitude of other components. If the same construction as employed for the horizontal laser 10 of FIG. 3 were to be utilized for a vertically oriented laser that would produce a laser beam of like power normal to the surface 24 of the semiconductor material 26 while occupying less area, it would have to project above the surface 24 as depicted in FIG. 4 and, therefore, would be nearly impossible to make and worthless with respect to planar integration. By changing the parameters and building a short cavity laser with similar mirrors and low area which is flush with the surface 24 of the semiconductor material 26 as depicted in FIG. 5, given the power available to drive it, the laser threshold level would be high and the power level of the resultant laser light 18 would be so low as to be virtually worthless. Thus, no practical method for constructing a surface-emitting laser with a useful power level output as part of OEICs presently exists in the art.
Wherefore, it is the object of the present invention to provide such a surface-emitting laser with a useful power level output which can be easily incorporated into OEICs employing presently available assembly apparatus and technology.
It is another object of the present invention to provide a surface-normal Distributed-Bragg-Reflector laser which utilizes parallel connections to a nipinipi . . . active region with periodically spaced i-gain segments and quarter-wave mirror stacks such that threshold currents and output powers are comparable to in-plane lasers of the same junction area.
Other objects and benefits of the present invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.